The disposal of waste water contaminated with hydrocarbon fuels, such as oil, is at present a problem. Oil-water separators are used to clean waste water by removing and retaining oil, allowing mostly purified water to be discharged into a drainage system. Such separators operate on the gravity differential principle and utilize the natural buoyancy of oil droplets in the water to force them to the surface, where they accumulate, effectively removed from the flow. Incoming contaminated water displaces clean water, already stripped of oil, and pushes it out of the vessel from the bottom through a discharge riser. Oil is retained at the top of the separator, where it may be periodically removed. Heavy, solid particles carried into the separator with the flow may have oil adhering to their surfaces; these particles settle to the bottom of the vessel as sludge and are also retained for periodic removal. Such separators are well suited for bulk fuel plants, truck stops, gas bars, petroleum terminals, industrial sites, vehicle repair garages, car washes, parking lots, rail and ship yards, and the like.
It is important to maintain the flow of water inside the tank from one end to the other as laminar as possible since turbulence causes oil droplets to mix in the vertical direction at a rate greater than the rise rate of the droplets.